Automotive suspension control system with road-condition-dependent damping characteristics

ABSTRACT

A suspension control system for automotive vehicles automatically adjusts the damping strength of variable shock absorbers or other dampers in accordance with road surface conditions as recognized by frequency analysis of a vehicle height or vibration sensor signal. The sensor signal reflects vertical displacement of the vehicle body from the road surface and includes high-frequency components due solely to displacement of the wheels of unsprung mass relative to the road surface and low-frequency components due to displacement of the vehicle body or sprung mass. The sensor signal is filtered into these separate frequency bands, the amplitude of each of which is compared to a corresponding reference level. The results of comparison give an indication of the degree and scale of irregularities in the road surface; specifically, a high-amplitude low-frequency component indicates larger-scale bumps and dips capable of bouncing the vehicle whereas a strong high-frequency component reflects a rough-textured road surface, such as a gravel. The comparison information is sent to a suspension system controller which causes actuation of the shock abosrbers to a stiffer mode of operation when the low-frequency sensor signal components are relatively strong.

This application is a continuation of U.S. application Ser. No. 691,531filed Jan. 15, 1985, now U.S. Pat. No. 4,770,438.

BACKGROUND OF THE INVENTION

The present invention relates generally to a suspension control systemfor an automotive vehicle with variable damping force depending uponroad surface condition. More specifically, the invention relates to asuspension control system which includes a sensor monitoring roadsurface conditions for use in controlling the stiffness of thesuspension in accordance therewith.

Various uses of road preview sensors have been proposed and developed.For example, SAE Technical Paper Series Nos. 680750 and 800520,respectively published on Oct., 1968 and Feb., 1980 show road previewsensors for use in suspension systems for obtaining optimum ridingcomfort and drivability. In addition, Japanese Patent First PublicationNo. 57-172808, published on Oct. 23, 1982 discloses a vehicle heightcontrol system which includes a sensor which detects rough roadconditions and adjusts the vehicle height level depending upon roadsurface conditions. A vehicle height or level sensor is employed in thedisclosed vehicle height control system for monitoring the relativedisplacement between the vehicle body and wheel axle. The output of thevehicle level sensor is compared with a reference level, which serves asa rough road criterion, and adjusts the vehicle height according to theresult of judgement of the road surface conditions.

In another example, Japanese Patent First Publication No. 58-30542,published on Feb. 23, 1983, discloses a variable damping force shockabsorber with damping characteristics varying in accordance with vehicledriving conditions. In the disclosed system, the magnitude of relativedisplacement between the vehicle body and wheel axle is measured and avehicle height variation indicative signal is derived from the measureddisplacement and the instantaneous vehicle speed. The vehicle heightvariation indicative signal value is compared with a reference valuewhich serves as a staff suspension criterion for adjustment of thedamping characteristics of the shock absorber in accordance therewith.

Conventional suspension control systems encounter difficulty inrecognizing the nature of vibrations causing relative displacementbetween the vehicle body and the wheel axle. For instance, when the roadwheel vibrates due to small-scale irregularities in the road surface, asofter or weaker suspension may be preferred in order to providesufficient riding comfort. On the other hand, when the vehicle bodyvibrates on a larger scale, i.e. if it starts to roll or pitch, astiffer suspension is preferable to provide riding comfort and betterdrivability.

SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide a suspensioncontrol system which overcome drawbacks in the prior art and can provideboth riding comfort and drivability by recognizing whether relativedisplacement between vehicle body and road wheels is due to vibration ofthe wheel or of the body relative to the plane of the road.

A more specific object of the present invention is to provide asuspension control system which includes a sensor capable ofdistinguishing between road surface conditions which cause road wheelvibrations and those which cause vehicle body vibrations so that thesuspension is stiffened only when the relative displacement due tovehicle body vibrations, such as rolling or pitching, is recognized.

In order to accomplish the aforementioned and other objects, asuspension control system, according to the invention, includes a sensorproducing a signal having an amplitude corresponding to the magnitude ofthe relative displacement between a vehicle body and a road wheel andhaving only frequency components corresponding to possible frequenciesof vibration of the vehicle body. A first comparator compares theamplitude of a predetermined first higher-frequency range of signalcomponents with a first reference level so as to detect the magnitude ofvibration of the road wheel. The first comparator produces a high-levelfirst comparator signal when the magnitude of the higher-frequency rangevibrations is greater than the first reference level. A secondcomparator compares the amplitude of a predetermined secondlower-frequency range of signal components with a second reference levelso as to detect the magnitude of vibrations of the vehicle body. Thesecond comparator produces a high-level second comparator signal whenthe magnitude of the lower frequency range vibrations is greater thanthe second reference level. A controller analyzes road surfaceconditions on the basis of at least the second comparator signal level.The controller produces a control signal which triggers avariable-damping-characteristics suspension mechanism to adjust thedamping characteristics between a stiffer suspension mode and a softersuspension mode depending upon the road surface conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings, throughout which like numerals refer to likeelements, may be of assistance in understanding the concepts behind thepresent invention and the structure, function and purpose of somepreferred embodiments thereof:

FIG. 1, a diagram of major elements of a typical vehicular suspensionsystem and of a first preferred embodiment of a suspension controlsystem;

FIG. 2, some examples of road surface sensor signals characteristic ofdistinct road conditions;

FIG. 3, a more detailed block diagram of the suspension control systemof FIG. 1;

FIG. 4, a more detailed block diagram of the controller of FIG. 3;

FIG. 5, a more detailed block diagram of the ultrasonic sensor of FIG.3;

FIG. 6, a flowchart of an ultrasonic sensor timing control programexecuted by the controller of FIGS. 3 and 4;

FIG. 7, a timing of some typical waveforms appearing in the circuitry ofFIG. 3;

FIG. 8, a diagram of possible states of a lowfrequency reference signal;

FIG. 9, a diagram of possible states of a highfrequency referencesignal;

FIG. 10, a longitudinal section through a shock absorber used in thefirst preferred embodiment;

FIG. 11, a partial longitudinal section through a modified shockabsorber;

FIG. 12, an enlarged longitudinal section through the damping forceadjusting mechanism of FIG. 11;

FIG. 13(A) and (B), cross-sections through the mechanism shown in FIG.12 at positions revealing the three possible fluid flow paths;

FIG. 14, an enlarged elevation in partial section of actuating elementsof the mechanism shown in FIG. 12;

FIG. 15, a block diagram similar to FIG. 3 of a second preferredembodiment;

FIG. 16, an elevation of a strut assembly for use with the secondpreferred embodiment;

FIG. 17, an enlarged elevation of part of the strut assembly of FIG. 16;

FIG. 18, a block diagram of a modification to the circuit of FIG. 15;

FIG. 19, a timing chart for typical signals obtaining in the circuit ofFIG. 18;

FIG. 20, a diagram of the possible states of a low-frequency referencesignal;

FIG. 21, a diagram of the possible states of a high-frequency referencesignal;

FIG. 22, a block diagram of another modification to the circuit of FIG.15;

FIG. 23, a block diagram of yet another modification to the circuit ofFIG. 15;

FIG. 24, a block diagram of still another modification to the circuit ofFIG. 15; and

FIG. 25, a diagram of a third embodiment of a suspension control system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, particularly to FIG. 1, the preferredembodiment of an electronic suspension control system in accordance withthe present invention generally comprises suspension strut assemblies10, each including a shock absorber 12 with variable shock-absorbingcharacteristics and a controller 100 adapted to produce a control signalfor actuating an actuator (not shown in FIG. 1) in each shock absorber12 in order to adjust the shock-absorbing characteristics in accordancewith the vehicle driving condition.

It should be appreciated that the term "shockabsorbing characteristics"used throughout the disclosure refers to the quantitative degree towhich a shock absorber produces damping force or spring force againstbounding and rebounding motion of the vehicle body as a sprung mass andthe road wheel assembly as unsprung mass, and pitching and rollingmovement of the vehicle body relative to the road wheel assembly. Inpractice, the shock-absorbing characteristics can be controlled invarious ways based on flow restriction between shock absorber workingchambers disposed in shock absorber cylinders. In the shown embodiment,the flow restriction is variable by means of a flow control valvedisposed within a reciprocable piston separating the chambers. Thepreferred embodiment described herebelow employs a shock absorber withtwo-way variable shockabsorbing characteristics, i.e. HARD mode and SOFTmode. Obviously, in HARD mode, the damping force generated in responseto bounding or rebounding shock applied to the vehicle is greater thanin SOFT mode. However, the shown embodiment is to be considered merelyas an example for facilitating better understanding of the invention andsimplification of the disclosure. In fact, shock absorbers which operatein three modes, i.e. HARD mode, SOFT mode and INTERMEDIATE or MEDIUMmode, are also applicable to the preferred embodiment of the suspensioncontrol system according to the invention. Some possible modificationsto the shock absorber will be disclosed together with the preferredshock absorber design given later.

Returning to FIG. 1, the controller 100 is connected to a road surfacesensor 200 which produces a sensor signal S_(r) indicative of roadsurface conditions, which will be referred to hereinafter as "roadsensor signal S_(r) ". The controller 100 may also be connected tosensors, such as a vehicle speed sensor, a brake switch, etc., in orderto receive the sensor signals indicative of the suspension controlparameters. The controller 100 is, in turn, connected to driver signalgenerators 102 which are responsive to the control signal from thecontroller, which control signal S_(c) can assume either of two states,namely HIGH and LOW. The driver signal generator 102 produces a drivesignal S_(d) which actuates the shock absorber to one of the HARD andSOFT modes.

The controller 100 is responsive to the road sensor signal S_(r) toproduce a control signal S_(c) for switching the shock absorber betweenHARD mode and SOFT mode. The general concepts of road surface-dependentsuspension control will be described herebelow with respect to FIGS.2(A) to 2(D). FIG. 2(A) shows the waveform of the road sensor signalS_(r) as the vehicle travels over a relatively smooth road. FIG. 2(B)shows a waveform of the road sensor signal as the vehicle moves along agraded but poorly surfaced road, such as a gravel road. FIG. 2(C) showsthe waveform of the road sensor signal as the vehicle travels along avery rough road. FIG. 2(D) shows the waveform of the road sensor signalas the vehicle travels along an undulant but well-surfaced road.

Generally speaking, softer or lower dampingforce characteristics arepreferable from the standpoint of good driving comfort. Thus, whentravelling along a relatively smooth road, the SOFT mode of the shockabsorber is preferable. In addition, in order to absorb relativelyhigh-frequency vibrations caused by an uneven road surface, a SOFTsuspension is preferred. On the other hand, when the vehicle istravelling on a relatively rough or undulant road, the vehicle body maytend to bounce due to abrupt vertical displacements. In this case, pitchsuppression becomes the most important factor for riding comfort anddriving stability.

It should be apparent that as the road wheel vibrates due to a roughroad surface, it generates high-frequency components in the road sensorsignal. On the other hand, large-scale vehicle body vibrations as inlateral rolling or vertical pitching motions are reflected in thelow-frequency components of the road sensor signal. Therefore, in theshown system, the road surface conditions, whether a relatively smoothroad (A), an uneven road (B), a relatively rough road (C) or an undulantroad (D), can be recognized by separately monitoring the high- andlow-frequency components of the road sensor signals.

FIGS. 3 to 10 show the first preferred embodiment of the suspensioncontrol system according to the present invention. FIG. 3 shows thecircuit layout of the first embodiment of the suspension control systemof the invention. The road sensor 200 comprises an ultra-sonic sensor202 which will be described later with reference to FIGS. 5 and 6, apair of band-pass filters 204 and 206, AC-DC converters 208 and 210, andcomparators 212 and 214. The band-pass filter 204 serves as low-passfilter for filtering the high-frequency components out of the outputsignal of the ultra-sonic sensor 202 (FIG. 7(A)) in order to pick uponly the lowfrequency components as shown in FIG. 7(B). The low-passfilter 204 is connected to the AC-DC converter 208. The AC-DC converter208 converts the output signal of the low-pass filter indicative of theamplitude of the lowfrequency components of the ultra-sonic sensoroutput signal into a corresponding direct-current signal as shown inFIG. 7(D). The AC-DC converter 208 is connected for output to thenon-inverting input terminal of the comparator 212. Similarly, theband-pass filter 206 is adapted to filter the low-frequency componentsout of the ultra-sonic sensor output signal so as to pick up only thehigh-frequency components as shown in FIG. 7(C). The output signal ofthe high-pass filter 206 is fed to the AC-DC converter 210. The AC-DCconverter 210 produces a direct-current signal shown in FIG. 7(E)indicative of the amplitude of the high-frequency components of theultra-sonic sensor output signal. The direct current level signal outputfrom the AC-DC converter 210 is applied to the comparator 214 throughthe non-inverting input terminal.

The inverting input terminals of the comparators 212 and 214 receiveinputs from reference signal generators 216 and 218 respectively. Thereference signal generator 216 has a voltage divider 220 consisting ofresistors R₁ and R₂. The junction 222 through which the reference signalgenerator 216 is connected to the inverting input terminal of thecomparator 212, is connected to the output terminal of the comparator214 via a diode D₂ and a resistor R₄. The junction 222 is also connectedto the controller 100 via an inverter 224, a diode D₁ and a resistor R₃for purposes discussed later. With this arrangement, the signal level Dof the reference signal produced by the reference signal generator 216varies depending upon the output of the comparator 214 and the signallevel C of control signal S_(c) from the controller. In practice, thereference signal level of the reference signal generator 216 can beobtained from the following equation:

    A=A.sub.0 -α.sub.1 ×C+α.sub.2 ×D   (1)

where

A is the signal level of the reference signal of the reference signalgenerator 216;

A₀ is the source voltage determined by the voltage divider 220 at thejunction 222;

α₁ is a constant determined by resistance value of the resistor R₃ ;

α₂ is a constant determined by resistance value of the resistor R₄.

C is a binary value determined by the control signal S_(c) ; and

D is a binary value determined by the output of the comparator 214.

Similarly, the reference signal generator 218 has a voltage divider 226consisting of a pair of resistors R₅ and R₆. A junction 228 between theresistors R₅ and R₆ is connected to the inverting input terminal of thecomparator 216. In addition, the junction 228 is connected to thecontroller 100 via the inverter 224, a diode D₃ and a resistor R₇. Withthis arrangement, the signal level of the reference signal produced bythe reference signal generator 218 varies depending upon the signallevel of the control signal from the controller 100. In practice, thesignal level of the reference signal of the reference signal generatorcan be obtained from the following equation:

    B=B.sub.0 -β×C                                  (2)

where

B is the signal level of the reference signal of the reference signalgenerator 218;

B₀ is the source voltage determined by voltage divider 226

β is a constant determined by the resistance value of the resistor R₇ ;and

C is a binary value determined by the control signal S_(c).

The controller 100 is connected for input from the output terminal ofthe comparator 212 and outputs a control signal value at either a HIGHlevel, by which the shock absorber is shifted to HARD mode, or at a LOWlevel shifting the shock absorber to the SOFT mode.

FIG. 4 shows the controller 100 which generally comprises amicroprocessor. In practice, the microprocessor performs controloperations not only depending upon the road surface conditions but alsodepending upon vehicle speed, other vehicle driving conditions, such asvehicle acceleration, and other preselected suspension controlparameters. One of these suspension control parameters, is the HIGH- orLOW-level output signal from the comparator 214, which switch thedamping characteristics of the shock absorber between the HARD and SOFTmodes respectively.

The microprocessor 100 generally comprises an input interface 102, CPU104, RAM 106, ROM 108 and output interface 110. In the shown embodiment,the microprocessor 100 is connected to the road sensor 200 via the inputinterface 102. The microprocessor 100 is also connected for input from aclock generator 112. RAM 106 includes a memory block 114 serving as amodeindicative flag F_(DH) which is set while the shock absorber isoperating in HARD mode. ROM 108 includes a memory block 116 holding theroad surface-dependent suspension control program as an interruptprogram triggered by a HIGH-level signal from the comparator 214. ROM108 also has a memory block 118 storing an ultra-sonic sensor controlprogram which triggers the ultra-sonic sensor at a given timing. Theoutput interface 110 of the microprocessor 100 is connected for outputof control signal S_(c) to each of the driver signal generators.

As shown in FIG. 5, the ultra-sonic sensor 202 comprises generally anultra-sonic wave transmitter 230 and a reflected ultra-sonic wavereceiver 232. The transmitter 230 is associated with the controller 100to receive therefrom a trigger signal S_(Tr) at a given timing. Thetransmitter 230 includes an ultra-sonic oscillator 234 and anultra-sonic wave transmitting section 236. The ultra-sonic oscillator234 is responsive to the trigger signal S_(Tr) from the controller 100,which is issued periodically or intermittently, to transmit or dischargeultra-sonic waves through the transmitter section 236 toward the roadsurface.

The ultra-sonic waves reflected by the road surface are received by areceiver section 238 of the receiver 232. The receiver section 238produces a receiver signal S_(Rc) having a value varying in accordancewith the amplitude of the received ultra-sonic waves. The receiversection 238 is connected to an amplifier 240 to supply the receiversignal S_(Rc) to the latter. The receiver signal S_(Rc) is amplified bythe amplifier 240 and transmitted to a rectifier 242. The rectifier 242is connected to the band-pass filters 204 and 206 as set forth above,through a shaping circuit 244. The rectifier 242 is also connected to apeak-hold circuit 246 which holds the peak value of the receiver signal.The peak-hold circuit 246 produces an analog peak-valueindicative signalS_(Pe) having a value proportional to the held peak value. The peak-holdcircuit 246 is connected for output to the controller 100 via ananalog-to-digital converter 248. The analog-to-digital converter 248outputs a binary signal indicative of the peak-valueindicative signalvalue to the controller 100.

The peak-hold circuit 246 is also connected to the controller 100 toreceive the trigger signal S_(Tr). The peak-hold circuit 246 isresponsive to the trigger signal from the controller to clear thecurrently held value.

FIG. 6 shows a timing control program executed by the controller 100 forcontrolling the trigger timing of the ultra-sonic sensor 200.

At the initial stage of execution of the timing control program, atrigger-signal-output-indicative flag F_(Tr) in a memory block 120 ofRAM is checked at a step 1002. The trigger signal F_(Tr) is set when thetrigger signal is output through the output interface 110 to thetransmitter 230 and is reset when the trigger signal is not beingoutput.

If the trigger signal-indicative flag F_(Tr) is set when checked at thestep 1002, then the timer value T₁ of a timer 122 in RAM is latched at astep 1004. The timer 122 continuously counts clock pulses from the clockgenerator 112. A trigger-signal-ON-time indicative time value t_(l) isadded to the latched timer value T_(l) at a step 1006. The resultantvalue (T_(l) + t_(l)), which serves as a trigger-signal-OFF time value,is transferred to and stored in a T₂ -register 124 in RAM 106, at a step1008. Then the flag F_(Tr) is set at a step 1010. A HIGH-level output isapplied to the output interface as trigger signal S_(Tr) at a step 1012.

During the period t_(l) starting from the time T_(l), the potential atthe output interface is held HIGH to continue application of the triggersignal S_(Tr) to the transmitter 230. The timer 122 continues countingthe clock pulses and produces a T_(l) -timer signal after period t_(l)which serves as a trigger signal for the timing control program.

In response to the T_(l) -timer signal at time T₂ marking the end of theperiod t_(l), the timing control program is executed again. Since thetrigger signal-indicative flat F_(Tr) was set at the step 1010 in theprevious cycle of program execution, the answer at the step 1002 becomes"NO". Thus, control passes to a step 1014 in which the timer value T₂ ofthe second timer 124 is accessed in RAM 106. Similarly to thefirst-mentioned timer 122, the timer 124 continuously counts clockpulses from the clock generator 112. An OFF-interval-indicative timedata t₂ is added to the latched timer value T₂ at a step 1016. The timedata t₂ has a value corresponding to a predetermined interval betweensuccessive trigger signals. The resultant time value (T₂ + t₂) is storedin the T_(l) -timer 122 of RAM 106 at a step 1018. Then, the flag F_(Tr)is reset at a step 1020. After the step 1020, the output level at theoutput interface drops to LOW to terminate transmission of the triggersignal to the transmitter, at a step 1022.

The detailed structure and operation of the aforementioned preferredembodiment of the ultra-sonic sensor has been disclosed in theco-pending U.S. patent application Ser. No. 650,705, filed Sept. 14,1984. The disclosure of the above-identified U.S. patent applicationSer. No. 650,705 is hereby incorporated by reference for the sake ofdisclosure.

The operation of the suspension control system as set forth above willbe described with reference to FIGS. 7 to 9. As the vehicle travels overa relatively smooth road as illustrated by the zone labelled "SMOOTH" inFIG. 7(A), the output signal of the ultra-sonic sensor 202 is rathersmooth and amplitudes of both the high- and low-frequency componentssmall and regular. The output of the ultra-sonic sensor 202 is filteredby the low-pass filter 204 and the high-pass filter 206 as respectivelyillustrated in FIGS. 7(B) and 7(C). Therefore, the output levels of theAC-DC converters 208 and 210 remain LOW, as shown in FIGS. 7(D) and7(E). The outputs of the AC-DC converters 208 and 210 are respectivelyinput to the non-inverting input terminals of the comparators 212 and214.

At this time, since the signal level of the control signal issued by thecontroller 100 remains LOW, as will become obvious later, the logicalvalue of the output of the inverter 224 become "1" (HIGH). The diodesD_(l) and D₃ are thus non-conductive. As a result, current flowingthrough the resistors R₃ and R₇ drops to zero. Therefore, the potentialat the junction 228 rises into correspondence with the divided powersource voltage B₀. As shown in FIG. 9, at this condition, the referencesignal level HL to be applied to the inverting input terminal of thecomparator 214 therefore becomes higher level. As a result, the logicalvalue of the output of the comparator 214 remains "0" (LOW).

Since, the diode D_(l) is cut off to block current flow through theresister R₃ and the logical value of the output of the comparator 214remains "0", the potential at the junction 222 corresponds to the powersource voltage as divided by the voltage divider 220, i.e. A₀, as shownby the solid line in FIG. 8. At this time, since amplitude oflow-frequency component of the output of the ultra-sonic sensor 202input to the non-inverting input terminal of the comparator 212 viaAC-DC converter 208 is smaller than the HARD/SOFT criterion representedby the reference voltage A₀, the output of the comparator 212 remainslow. Therefore, the control signal produced by the controller 100 isLOW, holding the shock absorber in SOFT mode.

As the vehicle starts to travel over a road full of dips and bumps asillustrated in the zone "UNDULANT" in FIG. 7(A), vibration of thevehicle body as a whole causing rolling and pitching increases while therelatively high-frequency vibrations of the road wheels remainsrelatively weak. Therefore, the amplitude of the high-frequencycomponent of the output of the ultra-sonic sensor 202 remains low. Onthe other hand, as vehicle body vibrations increase, the low-frequencycomponent of the ultra-sonic sensor output increases. Therefore, thewaveform of the output of the low-pass filter 204 becomes morepronounced as illustrated in FIG. 7(B). As a result, the output level ofthe AC-DC converter 208 jumps to a higher level as shown in FIG. 7(D).On the other hand, the output level of the AC-DC converter 210 remainslow as shown in FIG. 7(E).

The amplitude of the low-frequency component of the ultra-sonic sensorvaries according to the nature of the waviness of the road surface. Forinstance, when the peak-to-peak spacing of road features is much greaterthan their peak-to-trough vertical displacement, the vehicle bodyvibrations may be relatively weak. In this case, as the actual change inthe sensor-to-road distance is relatively small, the amplitude of thelow-frequency component of the ultra-sonic sensor 202 will remainrelatively low. On the other hand, if the spacing of the road featuresis relatively short in relation of the peak-to-trough height, thevehicle body vibrations may become significant. In this case, the rateof change of the sensor-to-road distance per unit time become greater,resulting in a relatively high-amplitude low-frequency component of theultra-sonic sensor signal.

Assuming that the road contours in the "UNDULANT" zone are sufficientlyabrupt to cause vehicle rolling and pitching to an extent in excess ofthe HARD/SOFT criterion represented by the reference signal from thereference signal generator 216, the output of the comparator 212 will goHIGH. The controller 100 is responsive to the HIGH-level comparatoroutput to produce a HIGH-level control signal and so operate the shockabsorber in HARD mode.

At this time, since the logical value of the control signal is "1"(HIGH), the input level to the reference signal generator 218 via theinverter 224 becomes logical value "0". The diode D₃ thus becomesconductive to allow some of the current available at the junction 228via the voltage divider of resistors R₅ and R₆ to drain through theresistor R₇. As a result, the output level of the reference signalgenerator 218 is lowered by a value BC, as shown in FIG. 9. At the sametime, the diode D_(l) in the reference signal generator 216 is alsoturned on by the LOW level input from the inverter 224. As a result,part of the current at the junction 222 is allowed to flow through theresistor R₃ and the diode D_(l). Since the high-frequency component ofthe ultra-sonic sensor 202 is still at a low amplitude and thus theoutput level of the comparator 214 remains LOW, the potential applied tothe junction 222 via the diode D₂ and the resistor R₄ remains nil. As aresult, the output level of the reference signal generator 214 as areference signal A is lowered by a value α_(l) C, as shown in solidlines in FIG. 8.

When employing the HARD mode of operation of the shock absorber,relative displacement between the vehicle body and the road wheel isinhibited to a greater degree than in the SOFT mode of operation of theshock absorber. This causes a reduction of the amplitude of theultra-sonic sensor signal S_(r) in comparison with that obtaining inSOFT mode. This down-shift of the sensor level can be compensated for bylowering the reference signal level by a value corresponding to thereduction in the amplitude of the sensor signal level due to HARD modeoperation.

In a zone labelled "ROUGH", small-scale irregularities in of the roadsurface increase in addition to the waviness of the road surface. As aresult, the road wheels vibrate at relatively high frequencies and thevehicle body rolls and pitches due to the waviness of the road bed.

Both the high- and low-frequency components of the ultra-sonic sensor202 are increased due to the overall roughness of the road surface.Since the lowfrequency component of the ultra-sonic sensor outputremains relatively strong, the output level of the comparator 212 and ofthe controller 100 remain HIGH, ordering continued HARD-mode operationof the shock absorber.

As set forth above, a HIGH-level control signal results in current drainvia the diodes D_(l) and D₃ of respective reference signal generators216 and 218. In addition, the increase in the amplitude of thehigh-frequency component of the ultra-sonic sensor 202 means that theinput level at the non-inverting input terminal of the comparator 218increases, as shown in FIG. 7(E). When the input level at thenon-inverting input terminal becomes greater than the reference signallevel B(=B₀ -βC) of the reference signal generator 218, the output levelof the comparator 214 goes HIGH. The HIGHlevel comparator output isapplied to the junction 222 of the reference signal generator 216 viathe diode D₂ and the resistor R₄. Therefore, the reference signal levelof the reference signal generator 216 increases by an amount α₂ D, tothe level shown in broken line in FIG. 8. This increase in the referencesignal level of the reference signal generator 216 applied to thecomparator 212 prevents the road wheel vibrations from influencingrecognition of the road surface waviness.

FIG. 10 shows the detailed structure of a variable-damping-force shockabsorber 12 employed in the first embodiment of the suspension controlsystem according to the present invention. The shock absorber 12generally comprises inner and outer hollow cylinders 20 and 22 arrangedcoaxially, and a piston 24 fitting flush within the hollow interior ofthe inner cylinder 20. The piston 24 defines upper and lower fluidchambers 26 and 28 within the inner cylinder 20. The inner and outercylinders define an annular fluid reservoir chamber 30.

The piston 24 is connected to the vehicle body (not shown) by means of apiston rod which is generally referred to by the reference number 32.The piston rod 32 comprises upper and lower segments 34 and 36. Theupper segment 34 is formed with an axially extending through opening 38.The lower end of the through opening 38 opens into a recess 40 definedon the lower end of the upper segment 34. On the other hand, the lowersegment 36 has an upper section 42 engageable to the recess 40 to definetherein a hollow space 44. An actuator is disposed within the space 44.The actuator 46 is connected to the driver circuit 16 through a lead 48extending through the through opening 38. The actuator 46 is associatedwith a movable valve body 50 which has a lower end extension 52 insertedinto a guide opening 54 defined in the lower segment 36. The guideopening 54 extends across a fluid passage 56 defined through the lowersegment 36 for fluid communication between the upper and lower fluidchambers 26 and 28.

The fluid passage 56 serves as a bypass for flow-restrictive fluidpassages 58 and 60 formed in the piston 24. The upper end of the fluidpassage 58 is closed by a resilient flow-restricting valve 62.Similarly, the lower end of the fluid passage 60 is closed by aflow-restricting valve 64. The flowrestricting valves 62 and 64 serve ascheck valves for establishing one-way fluid communication in oppositedirections. In addition, since the flow-restriction valves 62 and 64 arebiased toward the ends of the fluid passages 58 and 60, they open toallow fluid communication between the upper and lower fluid chambers 26and 28 only when the fluid pressure difference between the upper andlower chambers 26 and 28 overcomes the effective pressure of the valves.

The cross-sectional area of the fluid passages 58 and 60 and the setpressures of the fluid-restriction valves 60 and 62 determine thedamping force produced in HIGH damping force mode. The cross-sectionalarea of the fluid passage 56 determines the drop in the damping force inthe LOW damping force mode in comparison with that in the HIGH dampingforce mode.

The movable valve body 50 is normally biased upwards by means of a coilspring 51. As a result, when the actuator 46 is not energized, the lowerend section 52 of the valve body 50 is separated from the fluid passage56 to allow fluid communication between the upper and lower chamber.When the actuator 46 is energized, the valve body 50 moves downwardsagainst the resilient force of the coil spring 51 to block the fluidpassage 56 with the lower end extension 52. As a result, fluidcommunication between the upper and lower fluid chambers 26 and 28 viathe fluid passage 56 is blocked. When fluid communication through thefluid passage is permitted, the damping force produced by the shockabsorber 14 remains LOW. On the other hand, when the fluid passage 56 isshut, fluid flow rate is reduced, thus increasing the damping forceproduced. Therefore, when the valve body 50 is shifted to the loweredposition, the shock absorber works in HIGH damping force mode to producea higher damping force against vertical shocks.

A bottom valve 66 is installed between the lower fluid chamber 28 andthe fluid reservoir chamber 30. The bottom valve 66 is secured to thelower end of the inner cylinder and includes fluid passages 68 and 70.The upper end of the fluid passage 68 is closed by a flow-restrictionvalve 72. The lower end of the fluid passage 70 is closed by aflow-restriction valve 74.

In the normal state wherein the control signal of the controller 100remains LOW, the movable valve body 50 is held in its upper position bythe effect of the spring force 51 so that the lower end extension 52does not project into the fluid passage 56. Therefore, the fluidcommunication is established through both the fluid passage 56 and theapplicable one of the flow-restricting fluid passages 58 and 60. As aresult, the total flow restriction is relatively weak to allow the shockabsorber to operate in SOFT mode.

In response to a HIGH-level control signal from the controller 100, thedriver signal generator 102 corresponding to each shock absorber 12becomes active to energize the actuator 46. The actuator 46 drives themovable valve body 50 downward. This downward movement shifts the lowerend of the extension 52 of the valve body 50 into the fluid passage 56so as to block fluid communication between the upper and lower fluidchambers 26 and 28 via the fluid passage 56. Therefore, the fluid canflow between the upper and lower chambers 26 and 28 only through one ofthe fluid passages 58 and 60. The fluid flow restriction is, thus,increased, resulting in a greater damping force than is produced in theSOFT mode. In other words, the shock absorber 12 operates in HARD mode.

FIGS. 11 to 14 show a modified form of thevariable-damping-characteristic shock absorber of FIG. 10. In thismodification, the shock absorber 12 can be operated in any of threemodes, namely HARD mode, SOFT mode and MEDIUM mode, in the last of whichdamping characteristics intermediate to those of HARD mode and SOFT modeare achieved.

The hydraulic shock absorber 12 has coaxial inner and outer cylinders302 and 304. Top and bottom ends of the cylinders 302 and 304 areplugged with fittings 306 and 305. The fitting 306 includes a seal 307which establishes a liquid-tight seal. A piston rod 308 extends throughan opening 312 formed in the fitting 306 and is rigidly connected to avehicle body (not shown) at its top end. The piston rod 308 is, in turn,connected to a piston 314 reciprocally housed within the inner cylinder302 and defining upper and lower fluid chambers 316 and 318 therein.

The piston 314 has fluid passages 320 and 322 connecting the upper andlower fluid chambers 316 and 318. The piston 214 also has annulargrooves 324 and 326 along its upper and lower surfaces concentric aboutits axis. The upper end of the fluid passage 320 opens into the groove324. On the other hand, the lower end of the fluid passage 322 opensinto the groove 326. Upper and lower check valves 328 and 330 areprovided opposite the grooves 324 and 326 respectively to close thegrooves when in their closed positions. The lower end of the fluidpassage 320 opens onto the lower surface of the piston at a pointoutside of the check valve 330. Likewise the upper end of the fluidpassage 322 opens onto the upper surface of the piston at a pointoutside of the check valve 328.

Therefore, the fluid passage 322 is active during the piston expansionstroke, i.e. during rebound of the shock absorber. At this time, thecheck valve 328 prevents fluid flow through the fluid passage 320. Onthe other hand, during the piston compression stroke, i.e. duringbounding movement of the suspension, the fluid passage 320 is active,allowing fluid flow from the lower fluid chamber 318 to the upper fluidchamber 316 and the fluid passage 322 is blocked by the check valve 330.

The piston rod 308 has a hollow cylindrical shape so that a dampingforce adjusting mechanism, which will be referred to generally by thereference numeral "400" hereafter, can be housed therein. The dampingforce adjusting mechanism 400 includes a valve mechanism 402 foradjusting the cross-sectional area through which the working fluid canflow between the upper and lower chambers. In the preferred embodiment,the valve mechanism 402 allows three steps of variation of the dampingforce, i.e., HARD mode, MEDIUM mode and SOFT mode, the narrowestcross-sectional area representing the HARD mode, the widest the SOFTmode and intermediate the MEDIUM mode. Although the preferred embodimentof the invention will be described hereafter in terms of a three-way,adjustable-damping-force shock absorber, the number of adjustablepositions of the shock absorber may be increased or decreased as desiredand is not limited to this example.

As shown in FIG. 12, the piston rod 308 defines an axially extendingthrough opening 404 with the lower end opening into the lower fluidchamber 318. A fitting 408 seals the lower end of the opening 404 of thepiston rod and has axially extending through opening 410, the axis ofwhich is parallel to the axis of the through opening 404 of the pistonrod. Thus, the through openings 404 and 410 constitute a fluid path 412extending through the piston rod. The piston rod 308 also has one ormore radially extending orifices or openings 414 opening into the upperfluid chamber 316. Thus, the upper and lower fluid chambers 316 and 318are in communication through the fluid path 412 and the radial orifices414.

A stationary valve member 416 with a flaring upper end 418 is insertedinto the through opening 404 of the piston rod. The outer periphery ofthe flaring end 418 of the stationary valve member 416 is in sealingcontact with the internal periphery of the through opening. Thestationary valve member 416 has a portion 420 with a smaller diameterthan that of the upper end 418 and so as to define an annular chamber422 in conjunction with the inner periphery of the through opening 404of the piston rod. The stationary valve member 416 has two sets ofradially extending orifices 424 and 426 and an internal space 428. Theradially extending orifices 424 and 426 establish communication betweenthe internal space 428 and the annular chamber 422. A movable or rotaryvalve member 430 is disposed within the internal space 428 of thestationary valve member 416. The outer periphery of the rotary valvemember 430 slidingly and sealingly contacts the inner surface of thestationary valve member 416 to establish a liquid-tight sealtherebetween. Radially extending orifices 432 and 434 are defined in therotary valve member 430 at positions opposite the orifices 424 and 426of the stationary valve member 416.

As shown in FIGS. 13(A) and 13(B), the orifices 424 and 426 respectivelyinclude first, second and third orifices 424a, 424b, 424c, and 426a,426b, and 426c. The first orifices 424a and 426a have the narrowestcrosssections and the orifices 432 and 434 are adapted to be inalignment with the first orifices to establish fluid communicationbetween the upper and lower fluid chambers 316 and 318 in the case ofthe HARD mode. The third orifices 424c and 426c have the widestcross-sections and the orifices 432 and 434 are adapted to be inalignment with the third orifices in the case of the SOFT mode. Thecross-sections of the second orifices 424b and 426c are intermediatethose of the first and third orifices and the orifices 432 and 434 areadapted to align therewith in the case of the MEDIUM mode.

A check valve 436 is provided within an internal space of the rotaryvalve member 430. The check valve 436 is normally biased towards a valveseat 438 by means of a bias spring 440 for allowing one-way fluid flowfrom the lower fluid chamber to the upper fluid chamber. This cause thebound damping force to be somewhat weaker than the rebound dampingforce.

The rotary valve member 430 is associated with an electrically operableactuator such as an electrical step motor 442 through a differentialgear unit 444 and an output shaft 446 as shown in FIG. 14. Apotentiometer 448 is associated with the output shaft 446. Thepotentiometer 448 includes a movable contact 450 with contactors 450a,450b and 450c. The contactors 450a, 450b and 450c are adapted toslidingly contact stationary contact elements 452a, 452b and 452c of astationary contact 452. According to the electrical connections betweenthe movable contact and the stationary contact, the potentiometer 448produces a mode signal representative of the rotary valve position andthus indicative of the selected mode of the damping force adjustingmechanism. The step motor 442 is electrically connected to a controller100 to receive the control signal as a mode selector signal which drivethe motor 442 through an angle corresponding to the rotary valvemovement to the corresponding valve position. In this case, thepotentiometer will return the mode signal as a feedback signal toindicate the instantaneous valve position.

It should be appreciated that the controller 100 may be operated eitherin automatic mode or in manual mode.

Returning to FIG. 11, the shock absorber has a fluid reservoir chamber332 between its inner and outer cylinders 302 and 304, which fluidreservoir chamber 332 is in communication with the lower fluid chamber318 via the bottom fitting 305 described previously. The bottom fitting305 may serve to produce damping force in cooperation with the pistonand the damping force adjusting mechanism during bounding and reboundingmotion of the vehicle. A relatively low pressure pneumatic chamber 336is also defined between the inner and outer cylinders 302 and 304.

The operation of the damping force adjusting mechanism 400 will bebriefly described herebelow with reference to FIGS. 13. FIGS. 13(A) and13(B) show the case of the HARD mode. In this case, the orifice 432 ofthe rotary valve 430 is in alignment with the orifice 424a and theorifice 434 is in alignment with the orifice 426a. During vehiclerebounding motion, i.e., in the piston compression stroke, the fluidflows from the upper fluid chamber 316 to the lower fluid chamber 318though the orifice 426a. On the other hand, during vehicle boundingmotion, the fluid flows from the lower fluid chamber 318 to the upperfluid chamber 316 through orifices 424a and 426a. Since the firstorifices 424a and 426a are the narrowest, the damping force produced inthis mode is the highest among the three selectable modes.

In case of the MEDIUM mode, the orifices 432 and 434 of the rotary valvemember 430 are respectively in alignment with the second orifices 424band 426b.

In case of the SOFT mode, the orifices 432 and 434 align with the thirdorifices 424c and 426c, respectively to cause fluid flow. Since thethird orifices 424c and 426c are the widest of the three sets, asdescribed above, the damping force created in this SOFT mode is thelowest.

According to the shown embodiment, the electric step motor 442 isconnected to the controller 100 through the driver circuit 16. Similarlyto the case of the two-way shock absorber, the controller 100 selectsany appropriate damping force state in accordance with detected roadsurface conditions but in this case produces a three-way control signalfor actuating the shock absorber to one of the SOFT, MEDIUM and HARDmodes. The driver circuit 16 is responsive to the control signal todrive the step motor 442 to operate the rotary valve member 430 to thecorresponding valve position.

As an alternative in the modification set forth above, only SOFT andMEDIUM modes may be used for roadcondition-dependent suspension control.Therefore, when the HARD mode is selected in the foregoing firstembodiment set forth above the controller 100 actuates the shockabsorber to MEDIUM mode.

FIGS. 15 to 17 show a second embodiment of the suspension control systemaccording to the present invention. This second embodiment also employsthe variable damping force shock absorber identical to that disclosedwith respect to the first embodiment of the invention. On the otherhand, this embodiment employs a vibration sensor for detecting therelative displacement between the vehicle body and the road wheel axle,instead of the ultra-sonic sensor employed in the first embodiment.

The vibration sensor 500 is associated with a shock absorber in order tomonitor axial displacement of the piston rod thereof. The vibrationsensor 500 produces an AC vibration sensor signal with a valuerepresentative of the relative displacement between the vehicle body andthe road wheel axle. The vibration sensor signal is fed to ahigh-frequency filter circuit 502 and a low-frequency filter circuit504. The high-frequency filter circuit 502 is adapted to remove thehigh-frequency components from the vibration sensor signal and pass onlythe low-frequency components thereof. The high-frequency filter circuit502 outputs a direct-current signal representative of the magnitude ofvibration of the vehicle body, which signal will be referred tohereafter as "low-frequency component indicative signal". Thelow-frequency filter circuit 504 is adapted to remove the low-frequencycomponents from the vibration sensor signal and pass only thehighfrequency components thereof. The low-frequency filter circuit 504produces a direct-current signal representative of the magnitude ofhigh-frequency vibrations of the road wheel axle, which signal will bereferred to hereafter as "high-frequency component indicative signal".

The low-frequency component indicative signal from the high-frequencyfilter circuit 502 is input to a comparator 506 through itsnon-inverting input terminal. Similarly, the high-frequency componentindicative signal of the high-frequency component filter circuit 504 isinput to a comparator 508 through its non-inverting input terminal. Eachof the comparators 506 and 508 has inverting input terminals connectedto a corresponding reference signal generator 510 or 512. On the otherhand, the comparator 506 has the output terminal connected to acontroller 514 which is substantially the same as set forth with respectto FIG. 4 and produces a control signal to operate the shock absorberbetween HARD and SOFT modes. The output terminal of the comparator 508is connected to the reference signal generator 510.

The reference signal generator 510 has a voltage divider 516 includingresistors R₁₀ and R_(ll) which generates a predetermined voltage at thejunction 518 between the resistors. Through the junction 518, thereference signal generator 510 is connected to the inverting inputterminal of the comparator 506. The junction 518 is also connected tothe controller 514 via a diode D₁₀ and a resistor R₁₂ to receivetherefrom the control signal. Also, the junction 518 is connected to theoutput terminal of the comparator 506 via a diode D_(ll) and a resistorR₁₃. On the other hand, the reference signal generator 512 has a voltagedivider 520 including resistors R₁₄ and R₁₅ with a junction 522therebetween. The junction 522 is connected to the controller 514through a diode D₁₂ and a resistor R₁₆.

In comparison with the circuitry of the first embodiment of suspensioncontrol system, the inverter is omitted and the polarity of the diodesbetween the junctions and the controller is reversed. This is due to thefact that, since the sensitivity of the vibration sensor is boosted bythe greater damping force produced in the HARD mode of shock absorberoperation, compensating by increasing the reference value is necessaryachieved for uniform detection of the HARD suspension criterion.

FIGS. 20 and 21 illustrate the behavior of the reference signals at thejunctions 518 and 522 respectively in response to extremes of roadsurface condition. When the road is relatively smooth, the comparators506, 508 and the controller 514 all output low-level signal due to therelatively low-amplitude sensor inputs. Thus, since neither referencesignal generator 510, 512 receives a boost from the controller, and thegenerator 510 similarly is not boosted by the comparator 508 via diodeD_(ll), the divider voltages E_(o), F_(o) are applied withoutmodification to the inverting input terminals of the correspondingcomparators 506, 508.

If the road surface becomes noticeably bumpier, the comparators 506, 508output HIGH-level signals. Thus, the first reference generator 510 mayreceive one boost G from the comparator 508 but certainly receives aboost H from the controller 514. The second reference generator 512receives a single boost I from the controller 514. The boost G isapplied to the reference voltage at junction 518 when high-frequencywheel vibrations are detected and the comparator outputs a HIGH-levelsignal.

FIG. 16 shows a suspension strut assembly employed in the secondembodiment of suspension control system according to the presentinvention, of FIG. 15. The strut assembly includes a shock absorber 524having variable damping characteristics and operable in either HARD orSOFT mode. A piston rod 526 extends from the shock absorber cylinder 528and is connected to a strut housing 530 of the vehicle body through amounting bracket 532. A rubber insulator 534 is interpositioned betweenthe mounting bracket 532 and the strut housing 530 for absorbingvibrations transmitted between the vehicle body and the shock absorber.

A suspension coil spring 536 is wound around the piston rod 526 of theshock absorber. The lower end of the suspension coil spring 536 seats ona lower spring seat 538 fixed to the outer periphery of the outer shockabsorber cylinder. On the other hand, the upper end of the suspensioncoil spring 536 seats on an upper spring seat 540 which is connected tothe mounting bracket 532 via a bearing assembly 542. The bearingassembly 542 allows the strut assembly to pivot freely about -the pistonrod 526. The upper spring seat 540 is rigidly secured to a dustinsulator cover 544, to which the upper end of elastically deformablerubber dust insulator 546 is secured. The lower end of the dustinsulator 546 is secured to the outer periphery of the outer cylinder ofthe shock absorber.

A closure 548 with a connecting ring 550 is fitted to the bottom of theshock absorber cylinder. The shock absorber cylinder is connected to asuspension arm (not shown) via the connecting ring.

The vibration sensor 500 is inserted between the mounting bracket 532and the strut housing 530. As shown in FIG. 17, the vibration sensor 500has a pair of piezoelectric elements 552 and 554 sandwiching a terminalplate 556. The piezoelectric element 552 is fixed to a disc plate 558with an axially extending cylindrical section 560. The disc plate 558 isfixed to the mounting bracket 532 for motion therewith. On the otherhand, the piezoelectric element 554 is secured to a disc plate 562 whichis, conversely, fixed to the cover plate 544 which is fixedly mounted ona step 564 formed between the shaft 566 and the threaded end 568 of thepiston rod.

The cylindrical section 560 surrounds the threaded section 568 of thepiston rod.

A lead wire 570 extends from the terminal plate 556 through a rubberseal 572. The lead wire 570 is connected to the high-frequency filtercircuit 502 and the low-frequency filter circuit 504. The output of thevibration sensor 500, i.e., the vibration sensor signal, is thus fed tothe filter circuits 502 and 504 via the terminal plate 556 and the leadwire 570.

The vibration sensor 500 constructed as above is not responsive to thestatic load applied to the vehicle, which generally causes a lowering ofthe vehicle level, such as passengers and/or luggage. The vibrationsensor 500 is thus responsive only to dynamic loads due to verticaldisplacements and forces applied to the road wheel and vehicle body. Thevibration sensor signal is thus indicative solely of the dynamic loadapplied to the vibration sensor 500.

As shown in FIG. 18, the high-frequency filter circuit 502 mayalternatively comprise a low-pass filter 502-1, a high-pass filtercomponent 502-2, a full-wave rectifier 502-3 and a smoothing circuit502-4. The lowpass filter is adapted to remove high-frequency componentsfrom the vibration sensor signal and pass only the low-frequencycomponents. In the preferred embodiment, the low-pass filter 502-1removes signal component at frequencies above 3 to 5 Hz so as to resolvesignal components in the range of 1 to 2 Hz. The highpass filter 502-2is adapted to remove very-low-frequency components from the output ofthe low-pass filter for the sake of noise suppression. In practice, thehigh-pass filter 502-2 is intended to remove frequency components below0.2 to 0.3 Hz. Similarly, the low-frequency filter circuit 504 comprisesa high-pass filter 504-1, a fullwave rectifier 504-2 and a smoothingcircuit 504-3. The high-pass filter is, in practice, designed to filterout signal components at frequencies below 3 to 5 Hz so as to picksignal components in the frequency range of 12 to 13 Hz.

The operation of the aforementioned highfrequency filter circuit 502 andthe low-frequency filter circuit 504 will be described with reference toFIG. 19.

As shown in FIG. 19, the vibration sensor signal (A) is filtered by thelow-pass filter 502-1 and the high-pass filter 504-1, as illustrated in(F) and (B). The low-frequency component of FIG. 19(F) passes throughthe high-pass filter 502-2, leaving only the signal components in thefrequency range of 1 to 2 Hz. The output (G) of the high-pass filter isrectified by the full-wave rectifier 502-3 into the waveform of FIG.19(H). The output of the full-wave rectifier 502-3 is smoothed by thesmoothing circuit 502-4 into the waveform of FIG. 19(I).

On the other hand, the output of the high-pass filter 504-1 of FIG.19(B) is rectified by the full-wave rectifier 504-2 into the waveform ofFIG. 19(C). The rectifier output is smoothed by the smoothing circuit504-3 into the waveform of FIG. 19(D).

The smoothed signals, respectively representative of the amplitudes ofthe low-frequency component and the high-frequency component of thevibration sensor signal, are respectively fed to the correspondingcomparators 506 and 508.

In the foregoing second embodiment of FIG. 15, only the output of thecomparator 506 is input to the controller as aroad-surface-condition-indicative signal. However, it would be possibleto apply both of the comparator outputs to the controller so that thecontroller can fully recognize the road surface conditions from thecombination of the comparator outputs. In this case, the road surfaceconditions can be analyzed according to the following table:

    ______________________________________                                                           506       508                                              Road Condition     OUTPUT    OUTPUT                                           ______________________________________                                        ROUGH ROAD         HIGH      HIGH                                             POORLY SURFACED ROAD                                                                             LOW       HIGH                                             UNDULANT ROAD      HIGH      LOW                                              SMOOTH ROAD        LOW       LOW                                              ______________________________________                                    

In the practical suspension control performed by the controller 100, therecognized road surface condition can be used in substantially the samemanner as that disclosed with respect to the first embodiment.

FIG. 22 shows a modification to the road sensor in the second embodimentof suspension control system according to the invention. In thismodification, comparators 507 and 509 are added. The comparator 507 isconnected in parallel with the comparator 506 and its non-invertinginput terminal is connected for input from the smoothing circuit 502-4.The inverting input terminal of the comparator 507 is connected to areference signal generator 511. On the other hand, the comparator 509 isconnected in parallel with the comparator 508 and connected to thesmoothing circuit 504-3 at its non-inverting input terminal. Theinverting input terminal of the comparator 509 is connected to areference-signal generator 513. The reference signal generator 511 isadapted to produce a reference signal having a value smaller than thereference signal produced by the reference signal generator 510.Similarly, the reference signal generator 513 is adapted to produce areference signal having a value smaller than that of the referencesignal of the reference signal generator 512. With this arrangement, themagnitude of vibration of the road wheel and of the vehicle body can bemore precisely detected than in the system shown in FIG. 15.

The vibration magnitude of the road wheel and vehicle body can beresolved according to the following table:

    ______________________________________                                        Vibration Magnitude                                                           of Vehicle Body                                                                              506 OUTPUT  507 OUTPUT                                         ______________________________________                                        STRONG         HIGH        HIGH                                               INTERMEDIATE   LOW         HIGH                                               WEAK           LOW         LOW                                                ______________________________________                                    

    ______________________________________                                        Vibration Magnitude                                                           of Road Wheel  508 OUTPUT  509 OUTPUT                                         ______________________________________                                        STRONG         HIGH        HIGH                                               INTERMEDIATE   LOW         HIGH                                               WEAK           LOW         LOW                                                ______________________________________                                    

From the foregoing tables, road surface conditions may be more preciselydetected than in the previously described systems according to thefollowing table:

    ______________________________________                                                         Vibration Magnitude                                          Road Condition     Vehicle Body                                                                             Road Wheel                                      ______________________________________                                        SMOOTH ROAD        SMALL      SMALL                                           TEXTURED ROAD      SMALL      INTER-                                                                        MEDIATE                                         GRAVEL ROAD        INTER-     BIG                                                                MEDIATE                                                    ROUGH ROAD         BIG        BIG                                             STONE PAVING OR THE LIKE                                                                         BIG        INTER-                                                                        MEDIATE                                         BUMP               BIG        SMALL                                           ______________________________________                                    

Given more precise resolution of the road surface conditions, moreprecise suspension control can be performed in order to fit the hardnessof the shock absorber in close correspondence to actual road surfaceconditions.

FIG. 23 shows another modification of the second embodiment of thesuspension control system of FIG. 15. In this modification,analog-to-digital converters (A/D converters) 580 and 582 are connectedfor input from the smoothing circuits 502-4 and 504-3. The A/Dconverters convert the analog smoothing circuit outputs into binaryvalues representative of the amplitude of the smoothing circuit outputsfor direct application to the digital controller 100.

Although the second embodiment has been illustrated as employing avibration sensor to detect relative displacement between the vehiclebody and the road wheel, this may be replaced by a well-known vehiclebody height sensor, accelerometer, strain gauge or the like.

FIG. 24 shows a further modification to the second embodiment of thesuspension control system according to the present invention. In thismodification, a comparator 590 is connected to the fullwave rectifier502-3 to receive the rectified signal through its non-inverting inputterminal. The inverting input terminal of the comparator 590 isconnected to a reference signal generator 591. The reference signalgenerator 591 produces a reference signal having a value representativeof the hard-suspension criterion, e.g. vibrations due to the vehiclepassing over a bump or depression on the road surface. The outputterminal of the comparator 590 is connected to a trigger circuit 592which is adapted to output a LOW-level pulse for a given period of time,e.g. 10 ms, in response to the rising edge of a HIGH-level comparatoroutput. The trigger circuit 592 is connected for output to a NOR gate594 which also receives an input from the low-frequency filter circuit504. The output terminal of the NOR gate is connected to a retriggerablemonostable multivibrator 596.

The NOR gate 594 outputs a HIGH-level signal when the trigger signalfrom the trigger circuit 592 is HIGH and the input from the filtercircuit 504 remains LOW. The HIGH-level output of the NOR gate triggersthe monostable multivibrator 596 to make the latter output a LOW-levelsignal for a given period of time. The controller 100 receives theoutput of the monostable multivibrator 596 and issues a HIGH-levelcontrol signal to switch the shock absorber into HARD mode for theperiod of time for which the monostable multivibrator output remainsLOW.

With this arrangement, the suspension control system temporarily hardensthe shock absorber as the vehicle passes over a bump or a depression onthe road surface and exhibits good response characteristics.

FIG. 25 shows the third embodiment of the suspension control system inaccordance with the present invention. In this embodiment, a knownvehicle height control system is used for hardness control of thesuspension. Such vehicle height control 4,349,077 to Sekiguchi et al,U.S. Pat. No. 4,327,936 to Sekiguchi systems have been disclosed in U.S.Patent and European Patent First Publication No. 0 114 700, published onAug. 1, 1984, for example. Detailed constructions of the suspensionsystem with vehicle height control as disclosed in the above-referencedpublications are hereby incorporated by reference for the sake ofdisclosure.

In the shown system, an expandable and contractable pneumatic chamber600 is formed above the shock absorber 602. The pneumatic chamber 600 isconnected to a pressurized pneumatic fluid source 604. The fluid source604 comprises a compressor 606 for pressurizing a fluid such as air, areservoir tank 608 connected to the compressor 606 through an inductionvalve 610, and a pressure control valve 612. The pressure control valve612 and the induction valve 610 are connected to the driver signalgenerator 102 to be controlled thereby.

According to the shown embodiment, the driver circuit 102 is connectedto the controller 100. When energized by the driver signal, pressurecontrol valve 612 closes to block pneumatic fluid communication betweenthe pneumatic chamber 600 and the fluid reservoir 608. As a result, theeffective volume of the pneumatic chamber 600 is precisely that of thepneumatic chamber itself. Since the damping characteristics due to thepneumatic pressure in the pneumatic chamber is related to the effectivevolume of the pneumatic chamber and a smaller effective volume isachieved by blocking fluid communication between the pneumatic chamberand the fluid reservoir, the pneumatic chamber becomes relatively rigidin this case, providing a larger damping force in response to vehiclebody-chassis displacement.

On the other hand, in the normal valve position, the pressure controlvalve 612 opens to allow fluid communication between the pneumaticchamber and the fluid reservoir. As a result, the effective volumebecomes equal to the sum of the volumes of the pneumatic chamber and thefluid reservoir. By providing a larger effective volume, the dampingcharacteristics of the pneumatic chamber are weakened.

As set forth above, according to the present invention, the vehicularsuspension system can provide both riding comfort and good drivabilityby controlling hardness of the suspension depending upon the roadsurface conditions.

It should be noted that although the shown embodiments control thedamping force and/or rigidity of the suspension system by adjusting thedamping characteristics of the suspension strut assemblies, it wouldalso be possible to perform suspension control by adjusting the rigidityof a roll-stabilizer employed in the vehicle suspension. Such variablespring-force or damping-force stabilizers for vehicle suspension systemshave been illustrated in the co-pending U.S. patent application Ser. No.647,648, filed Sept. 6, 1984. The contents of the above-identifiedco-pending U.S. Patent Application are hereby incorporated by referencefor the sake of disclosure.

What is claimed is:
 1. A suspension control system for an automotive vehicle comprising:a damper means interpositioned between a vehicle body and a road wheel for absorbing, at least in part, relative displacement between the vehicle body and road wheel, said damper means having damper characteristics selectably variable between a soft mode and a hard mode; a sensor producing a sensor signal, the amplitude and frequency of which correspond to the amplitude and frequency of vibrations of the vehicle body; a filter means receiving said sensor signal and outputting a filter signal representative of the amplitude of a specific frequency of vibrations of the vehicle body; a comparator means receiving said filter signal and comparing the amplitude thereof with a reference value, said comparator means producing a comparator signal when the filter signal value is greater than the reference value; and a controller associated with said damper means for normally operating the latter in said soft mode, said controller being responsive to said comparator signal to operate said damper means in said hard mode. 